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THE BELL SYSTEM 

TECHNICAL JOURNAL 



, volume xliii January 1964 number 1, part 1 
*\ 

Copyright 1964, American Telephone and Telegraph Company 

The Ferreed 

By A. FEINER 

(Manuscript received August 20, 1963) 

The advantages of the fenced as a switching network cross-point led to an 
early decision to adopt it for use in electronic switching systems. The 
prospect of large-scale use of the device gave impetus to a search for an 
economical, easily fabricated component. This paper describes the con- 
siderations which influenced the choices of a suitable magnetic material, 
magnetic circuit geometry, and coil design that were made for the produc- 
tion model. 

I. INTRODUCTION 

The concept of the ferreed was presented in an earlier article in this 
journal. 1 The purpose of this paper is to describe the evolution of this 
device during its further development. 

To recollect, a ferreed is a device born of marriage between miniature 
sealed reed contacts (see Ref. 2) and an external magnetic circuit 
containing remanently magnetizable members. Operation or release of 
the sealed contacts can be controlled by setting the remanent members 
in one of two magnetic states by means of short current pulses. 

Among the several useful properties that can be brought about in 
the ferreeds by selection of the proper magnetic configurations and coil 
design Is the ability to respond to coordinate excitation — a vital re- 
quirement for any device considered for a network crosspoint. 

Recognition of the potential advantages of a switching network cross- 
point with metallic contacts, absence of holding power and the ability 
to operate in times much shorter than prior electromechanical devices 

1 



2 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 

led to an early decision to adopt it for the network of No. 1 ESS (Elec- 
tronic Switching System) — the new telephone switching system sched- 
uled for its commercial debut in 1965. 

The intended application of the ferreed in the switching network of 
No. 1 ESS, where it would appear in very large numbers (14-20 cross- 
points per line), gave impetus to a search for an economical, easily 
fabricated embodiment. Several important choices had to be made with 
regard to the geometry of the magnetic circuit, the winding configuration 
and the remanent magnetic material. At the same time, the require- 
ments of the sealed reed contact were reexamined, and a modified ver- 
sion of it known as the 237B contact was adopted for ferreed use. 

II. THE CROSSPOINT FERREED 

2.1 Choice of Remanent Material 

All original work on the ferreeds was based on the use of a specially 
developed cobalt ferrite as the remanent material. In time, certain 
inherent difficulties became apparent: notably, a strong temperature 
dependence of the magnetic properties and low flux density, leading to 
structures of large cross section and poor efficiency. Furthermore, as 
more thought was given to the ferreed as a system component, it was 
found that the originally postulated microsecond speeds for the actuation 
of the ferreed were neither required nor practical from the standpoint 
of driving requirements. 

These considerations opened the way to a search for a metallic sub- 
stitute. Several chromium and tungsten steel compositions were investi- 
gated and found wanting due to lack of squareness and fullness of the 
hysteresis loop — ■ properties whose importance were stressed in Ref . 1 . 

The attention soon centered on a recent addition to the list of cobalt- 
iron- vanadium alloys — Remendur. The name of this alloy refers to its 
primary magnetic characteristic, i.e., a remanence greater than 17,000 
gauss. This is coupled with a square hysteresis loop and a coercive force 
from 1 to 60 oersteds. With a nominal composition of 48 per cent cobalt, 
48 per cent iron, 3.5 per cent vanadium and 0.5 per cent manganese, 
Remendur bridges the gap between the high coercive force of Vicalloy 
and the low coercive force and high permeability properties of 2V- 
Permendur and Supermendur. Fig. 1 shows a hysteresis loop obtained 
on a Remendur strip developed for ferreed use. Of import ance to the 
ferreed application is the squareness B r /B s and fullness \ZH B /H c B r 



THE FERREED 




-20 20 

H IN OERSTEDS 



Fig. 1 — Hysteresis loop of Remendur used in ferreeds. 

of the hysteresis loop. This property implies that the energy expendi- 
ture in establishing a desired end state approaches a minimum, and 
that the excess flux generated in the same process is small — important 
in view of the interference problems present in ferreed arrays. 

2.2 Choice of Geometry 

There exist two basic forms of ferreed structures — the parallel and 
the series ferreeds. These are illustrated in Fig. 2. The choice of Remen- 
dur, the need for tight magnetic coupling between the remanent mem- 
bers and the reed contacts, and the relative ease of fabrication led to 
adoption of the series structure for the crosspoint ferreed. 

That structure is shown in Fig. 3 in the form used in the ESS network. 
Mounted on each side of the reed contacts, which are molded together 
in plastic to form a single piece part, and extending approximately over 
the length of the glass envelopes, are two flat plates of Remendur. 
Notches on the plastic and on the plates permit accurate relative posi- 
tioning of the two. 

The reeds and the remanent plates are inserted into plastic coil forms 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 




PARALLEL 
FERREED 




SERIES 

FERREED 



•s 

OPERATED 

Fig. 2 — Principles of parallel and series ferreeds. 

molded into a steel plate. This steel plate acts as a common shunt for 
the whole array — it divides each crosspoint magnetically into two 
separately controllable halves, greatly reducing the energy requirement 
for producing the release state in which, as shown in Fig. 4, the two 
halves of the remanent members are magnetized in opposing directions. 
The same steel plate acts as the mechanical backbone of the whole 
array. 

2.3 Coil Design 

The differential excitation mode was selected to provide coordinate 
addressing of crosspoints. Fig. 5 reviews this principle as applied to a 
series ferreed. Each crosspoint has two sets of windings — one for each 
coordinate. Each set contains a winding of N turns on one side of the 
shunt plate and one with a larger number, typically 2N, on the other 
side. The 2iV-turn winding is connected series opposing the A^-turn 
winding. One pair of windings is in series with the corresponding pairs 
of all crosspoints in the same row, while the other is in series with the 
pairs of all crosspoints in the same column of the array. As the paired 
windings oppose each other, energization produces the release state in 
every crosspoint energized, except the one where both pairs of windings 



THE FERREED 5 

are energized simultaneously — the crosspoint at the intersection of the 
energized row and the column. 

The logic inherent to differential excitation was found to be well 
suited to network array operation, in which, in general, only one cross- 
point in each row or column need be operated. 

No separate release actions are required, as operating a crosspoint 
automatically releases other crosspoints associated with the same row 
and column. 

The design of the coils has to take in account the energization re- 
quirements of a single crosspoint as well as the system requirement 



237 B SEALED CONTACTS 
2 PER MOLDED ASSEMBLY 



18-TURN COIL 



HORIZONTAL 
STRAPS 



COIL FORM- 




REMENDUR 
PLATES 



„--39-TURN COIL 



SHUNT PLATE 



Fig. 3 — Exploded view of the two-wire crosspoint ferreed. 



THE BELT, SYSTEM TECHNICAL JOURNAL, JANUARY 1904 




<j in ni — - <\j o 

saaisuBO ni Nouoauia ivixv 

3H± NI A1ISN31NI 01313 DI13N3VW 




J I L 



^ fi) (u - - ni pi t •o <o 

sa3isa3o ni Nouosaia ~ivixv 

3H1 NI A1ISN31NI Q13I3 0I13N3VW 



y\ v 



'i 1 



s 
x 



bi. 



THE FERREED 



MAGNETIC 
SHUNT 




(a) 



N 



X 0- 



^: 



KN 



SHUNT PLATE 



^. 



/■ 



\<L 



!~ 






KN 



K = 2 
N=19 



(b) 



Fig. 5 — Winding configuration for differential excitation of the series ferreed: 
(a) winding pattern, (b) mirror symbol notation. 



calling for simultaneous pulsing of 32 winding pairs in the process of 
establishing a connection through two stages of ferreed switches. 

In ESS, these considerations led to the adoption of coils with windings 
of 18 and 39 turns wound with 25-gauge copper wire. With these coils, 
the nominal operating current pulse of 10 amperes peak amplitude and 
250 microseconds duration insures adequate margins for both operation 
and release of the crosspoint. 

The coils are wound directly on the coil forms by a machine that 
winds eight rows (or columns) of crosspoints simultaneously in a con- 
tinuous succession, each with a single length of wire. This eliminates 



8 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 



soldered connections between coils, thus reducing the winding cost and 
improving the reliability of the assembly. 

The winding sense is reversed in adjacent crosspoints. This magnetic 
"checkerboarding" was found to be an effective means for reducing 
magnetic interaction phenomena as well as the noise pickup in the 
transmission pairs due to ferreed energizing pulses. 

2.4 Crosspoint Arrays 

Switching network considerations led to selection of an 8 X 8 cross- 
point array as a basic network building block. In Fig. 6, such an array 
is shown. In addition, specifically for the concentrating stages of the 
network, several other array types were required: a switch providing 
each of 16 input terminal pairs with an access to 4 out of 8 available 
outputs, and 8X4 and 4X4 switches. It was found that each of these 
arrays could be derived from the basic 8X8 apparatus unit by suitably 
changing the connections of the control windings and the voice-pair 
strappings. Fig. 7 shows these connections for all the developed ferreed 




Fig. 6 — An 8 X 8 ferreed switch with covers removed. 



THE FERREED 



( .) 



switch types. As can be expected, this standardization of the physical 
size and component parts of the switches has eased the manufacturing 
and the network equipment design problems. 

The connections shown between the ends of the row and column 
control winding chains stem from the access scheme adopted in the 
network design. In this scheme, identical current is applied to both 
coordinates by connecting them effectively in series when energizing a 
crosspoint at their intersection. 




\ — I T 




12 13 

J L 



1 1 I i- 



T T T T 



(a) 




Fig. 7 — Control winding interconnection for three types of two-wire switches: 
(a) 16 X 4/8, (b) 8X4, and (c) 8X8. 



10 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 

III. DESIGN TECHNIQUE 

When the problem of designing the ferreed was first approached, it 
was found that the usual lumped-constant, linear magnetic circuit 
approach, while sufficient to yield a workable device, did not provide 
the means for its optimization; neither did it give an assurance of 
margins in face of tolerance allowances that have to be made for the 
whole structure, and variations in reed contact properties and in the 
magnetic properties of Remendur. Several attempts were made to refine 
the analytical tools toward this end. While providing qualitative in- 
sight into the operation of the device, they were frustrated from attain- 
ing the ultimate goal of a quantitative, explicit solution by the complex- 
ity of the problem caused by the rather difficult geometry and the 
essential nonlinearity of the magnetic materials. 

As a result, the refinements in the ferreed design had to be based 
largely on experimental techniques. Over the years, numerous experi- 
mental ferreed study techniques have been devised. These include the 
use of search coils with integrators, hysteresis measurements of reeds 
and the remanent magnetic members, Hall probes in the crosspoint 
structure and the reed gap, and reversible permeability measurements 
of the reeds. Supplemented by experiments in which the component 
parts of the structure, their positioning and the driving conditions 
underwent systematic variations, these techniques were instrumental 
in arriving at the present structure. 

The use of Hall probes provided two study techniques. First, Hall 
probes were employed to measure longitudinal magnetic field intensity 
along the ferreed axis, after applying varying operate and release pulses. 
Second, via the use of specially constructed sealed reeds with Hall 
probes mounted in the gap of the reed, it was possible to measure the 
resultant magnetic flux density in the reed gap under varying operating 
conditions. The drawback of the techniques lies in the upsetting of the 
ferreed magnetic circuit by the absence of the reed or introduction of a 
permanently open reed structure. 

Reversible permeability measurements of the sealed reeds, accom- 
plished via inductance measurements of small sense coils at about 100 
kc, provided a convenient means of determining the instantaneous ap- 
plied mmf to the sealed reeds under varying operating and interference 
conditions. The technique was especially useful because it permitted 
the use of ordinary sealed reeds under actual operating conditions, and 
it was free of drift problems since no integrator circuits were involved. 
On the other hand, the nature of the reversible permeability character- 



THE FERREED 11 

istic of the sealed reed is so insensitive in the released state of the sealed 
reeds as to make its use not suitable in that region. 

IV. OTHER FERREED TYPES 

4.1 The Bipolar Ferreed 

In the process of designing a ferreed switching network, the need 
arose for a device containing a pair of contacts that would be indi- 
vidually controllable. A typical use for this device is disconnection of the 
line current sensing element at the line circuit whenever a connection 
is established in the switching network (cutoff relay function) . A postu- 
lated property of this device — to respond to control current pulse polarity 
to open or close its contacts — was found to permit integrating the con- 
trol access with the one for the crosspoints. 

An adaptation of the parallel ferreed principle, shown in Fig. 8, 
provided a suitable embodiment meeting this need. Of the two parallel 
remanent members, one consists of a permanent magnet material, 
Cunifc I; the other, surrounded by a single coil, of Remendur. Con- 
tact closure or release depends on the polarity of the current pulse 
applied to the coil. Eight such devices packaged together form a single 
apparatus unit compatible in its length with the crosspoint units. 

4.2 The Four-Wire Crosspoint Array 

For use in switching networks requiring two separate directions of 
transmission, the two-wire crosspoint design has been extended to 
permit the operation of four contacts at every crosspoint location. The 
four contacts are arranged in a square pattern and are surrounded by an 
open-ended box formed by four remanent plates. The windings are 
similar to those of the two-wire array and again an eight-by-eight size 
has been chosen; Fig. 9 shows an individual crosspoint and an over- 
all view of the unit. 

V. SUMMARY 

Out of the original concept of the ferreed originated a whole class of 
useful switching devices. Characterized by small size, high speed of 
operation and absence of holding power, they permit retaining the 
desirable aspects of metallic contacts in the environment of electronic 
switching machines without creating undue time compatibility problems. 



^a 



WINDING 



REMENDUR-- 




PERMANENT 
MAGNET 



X SEALED CONTACT 
SWITCHES 



(a) 




Fig. 8 — (a) The bipolar ferreed; (b) a 1 X 8 apparatus unit. 

12 



HORIZONTAL 
_ STRAPS 



14)237B SEALED 
CONTACTS — 
2 PER MOLDED 
ASSEMBLY 



19-TURN COIL 



COIL FORM 




VERTICAL 

STRAPS 




(b) 



Fig. !) — (a) Exploded view of a single four-wire crosspoint; (b) over-all view 
of an 8X8 switch with protective covers removed. 

13 



14 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1964 



Table I — Summary of Ferreed Characteristics 



Switch 


Dimensions (Inches) 


Operate and 
Release Pulse 


Contact Characteristics 


Code 


Type 


Height 


Width 


Length 


Peak 

Ampl. 

(A) 


Width 


Max. 
Res. 

(ohms) 


Max. 
Oper- 
ate 
Time 
(ms.) 


Max. 

Surge 
Cur- 
rent 


Life 


242 A 


2-wire 8X8 


6% 


2H 


9K 


9 


200 
to 
500 


0.2 


3 


3A* 




242 B 


2-wire (2) 8 X 4 




242 C 


2-wire 16 X 4/8 


2 X 10«f 


252 A 


4-wire 8X8 


9% 


2H 


9H 


9 


200 
to 
300 




241 B 


2-wire 1 X 8 


1% 


2H 


Wi 


6 


200 
to 
500 


5{ 


3 


3A 


2 X 10 a 



* To protect the contacts, crosspoints are operated and released in a dry cir- 
cuit — maximum surge current refers to current value applied to closed contacts, 
t Minimum life of 2 X 10 6 operations with contact resistance below 0.2 ohm. 
X This contact breaks a maximum of 40 ma in its operation. 

Table I gives a summary of the characteristics of the ferreed codes how 
in existence. 

VI. ACKNOWLEDGMENTS 

Many people have contributed important ideas and skills to make the 
ferreed a success ; the author would like to offer his particular apprecia- 
tion to Messrs. H. L. B. Gould and D. H. Wenny for their work on the 
Remendur, Messrs. R. L. Peek, F. H. Myers, and H. Raag for their 
work in magnetic design of the ferreed, and Messrs. H. J. Wirth and 
R. A. Billhardt for the mechanical design. 

The credit for solving the manufacturing problems should go to Mr. 
G. A. Mitchell of the Western Electric Company at Columbus. 



REFERENCES 



1. Feiner, A., Lovell, C. A., Lowry, T. N., and Ridinger, P. G., The Ferreed — 
A New Switching Device, B. S.T.J. , 39, January, 1960, p. 1. 

2. Keller, A. C, Recent Developments in Bell System Relays — Particularly 
Sealed-Contact and Miniature Relays, B. S.T.J. , this issue, p. 15.